Current flow in high voltage ac transmission systems is, typically, monitored by means of shunts or current transformers. The signals are, typically, derived using components which float at the power line voltage and must be transmitted to metering, control, and protective relaying equipment which operates near ground potential. Such signals have, in the prior art, been transmitted to ground through current transformers which, necessarily, included expensive and complex dielectric elements to provide necessary high voltage insulation.
The criteria for effectively monitoring current flow and providing protection to high voltage transmission lines requires that current flow data be transmitted from line voltage to ground over a channel having a dynamic range of from approximately one part in 10.sup.4 to approximately one part in 10.sup.5, and an absolute accuracy of approximately .+-.0.3 percent. This performance is obtainable with state-of-the-art current transformers. However, the size and cost of such transformers increases drastically with increased transmission line voltage and may be prohibitive for proposed extra high voltage transmission systems.
It has been proposed that a suitable data channel from a high voltage transmission line to ground potential might be provided by a light source/photodetector pair coupled with a dielectric light-transmission system: for example, a fiber optic light pipe. One embodiment of such a system included analog-to-digital converter circuitry operating at line potential to produce a binary representation of line current flow. The binary data was transmitted to ground along one or more optical fiber light pipes, for example by a pulse code modulation system, and was decoded at ground level to provide analog current flow data. Such systems can be constructed to provide high dynamic range, linearity, and accuracy. However, a large number of active electronic components in the transmitter are required to operate at line potential and the probability of failure in such systems is, therefore, high.
In many current monitoring applications, current flow data may be transmitted to ground through a channel comprising an intensity modulated light-emitting diode (LED) coupled to a photodetector. The ac line current signal is applied to a light-emitting diode operating at power line potential. Light output from the diode is transmitted along an optical fiber or similar light-transmission system to a silicon photodiode operating at ground potential which produces a voltage output in proportion to the light received. The light output of light-emitting diodes is, of course, a nonlinear function of applied current. In accordance with the teachings of the prior art, a silicon photodiode operating in a negative feedback loop may be utilized at the transmitter to linearize the light-emitting diode output and the channel response.
The linearity of silicon photodiodes which are utilized in the above-described optical channel remains relatively constant with temperature. However, the sensitivity of such photodiodes, as well as the electrical characteristics of other components in an intensity modulated data channel have been found to vary as a function of ambient temperature. For example, the sensitivity of silicon photodiodes has been found to vary by approximately .+-.8 percent over a 100.degree. C temperature range. Means must, therefore, be provided to eliminate or cancel drift in the absolute gain of system components before this intensity modulated LED may be effectively utilized to transmit current data in high voltage ac transmission systems.